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How "powerful" is a wing tip vortex?
I know that aircraft generate wake turbulence and wingtip vortices, and the strength of those phenomena are related to the size of the aircraft, but how powerful are wingtip vortices in terms a non-pilot would understand?
For example, if a human were standing inside of a wingtip vortex would it have the strength to knock them off their feet?  If the wingtip vortex were turned vertical like a tornado would it be an EF1, or an EF5?

Strong enough that recommended separation for a small plane following big iron on takeoff is five minutes.

Strong enough to disintegrate a small 4-seater aircraft or similar.

From this excellent answer by Peter Kämpf, I understand the wingtip vortex is very small compared to the downwash/wake vortex. I wonder if there is not a confusion between them in the answers and some comments.

@mins I see no confusion, except that in aviation the phenomenon that causes an aircraft to leave a "wake" is routinely and imprecisely referred to as, and attributed to, "wingtip vortices" (There are vortices, as is obvious from any number of photos and videos,  but as Peter correctly notes they're not entirely a "wingtip" phenomenon). The FAA term "Wake Turbulence" is a more complete and correct description of what makes the little plane go upside-down when it flies too closely behind the big plane :)

If we're talking about large aircraft the answer is Powerful enough that you want to stay out of them.
I refer you to this delightful FAA video in which a Cessna (looks like a 172) was flown into the wake of a C-5 Galaxy. The Cessna was rolled through 360 degrees by the vortex.
NASA research has shown that the rotational speed of wake vortices can exceed 150 miles per hour (130 knots).
While the Enhanced Fujita Scale rates tornadoes based on damage a 150 mile per hour wind speed is in the range estimated for an EF3 tornado ("Severe Damage", defined as Entire stories of well-constructed houses destroyed; severe damage to large buildings such as shopping malls; trains overturned; trees debarked; heavy cars lifted off the ground and thrown; structures with weak foundations are badly damaged.) - This should reduce your desire to be anywhere near the wake vortex of a large aircraft.

If we're talking about smaller aircraft (say two Cessnas flying in trail, and the trailing aircraft somehow gets below the lead) it's strong enough to be unpleasant - the wake from the leading aircraft will shake and rock the wings of the trailing aircraft, but it probably won't flip the trailing aircraft over.
Many pilots are familiar with encountering their own wake at some point in flight training: It can happen when practicing steep turns in calm air.
If the weather and timing of the turn are "just right" and you produced a circle rather than an oval yo will hit the descending wake from your outside wing as you roll level at the 360 degree mark. This produces a slight "bump" at the end of the maneuver (visible in this video).

Depending on the size of the aircraft, very powerful. One way of thinking about his would be that the velocities in the wake are directly related to the lift produced. So, heavier the aircraft, larger the lift and greater the velocities.
A human being inside a vortex of a large would feel like he's inside a tornado. For all practical purposes, it is a mini-tornado.
The maximum vortex strength occurs when the aircraft is heavy, slow and clean (with low wingspan). Wind speeds upto 240-300 kmph has been recorded in certain cases.
This is the main reason for having separation intervals between two aircraft taking off, especially after a 'heavy' aircraft. The best examples of what happens when someone is inside these wingtip vortices comes from incidents where (smaller) aircraft gets caught up in the wake of (larger) aircraft and it doesn't make for very happy reading.

In November 2008, a SAAB 340B got caught in the wake of an Airbus 380 conducting a parallel approach. During this incident, the ATSB report shows that the SAAB 340(which is not a small plane by any mens, having a wingspan of over 20m),

...experienced an uncommanded 52° roll to the left, in conjunction with an 8° nose-down pitching motion. Immediately after, the aircraft rolled through wings level to a 21° right bank angle. The aircraft also experienced an altitude loss of 300 to 400 ft in the 9 to 15-second period during which the crew regained control of the aircraft.

In the same year, a Learjet-45 flew into the wake turbulence of a 767-300 (separated by 4nm),lost control, killing everyone on board.

From these incidents, it is pretty clear that getting into the vortices, especially the large one is a very bad idea.


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Why don't airliners have bigger cockpit windows?
A bigger cockpit window would increase the pilot's vision of a runway. So why can't they be bigger?
Note that this question covers cabin window size already, but this question is about cockpit windows specifically.

Larger windows would be harder to make strong enough. I'm sure there's an engineering tradeoff somewhere.

For one, the 787 has pretty big windows...

hmm... It looks like that other question is concerned more with passenger cabin windows than flight deck windows. Having said that, flight deck windows are usually sufficiently large to see the runway just fine on pretty much all airliners. They usually wrap all the way around for at least 180 degrees of vision. And the runway is (hopefully) down rather than up, so making them taller wouldn't help (unless you reduce the size of the instrument panel and/or get rid of the radome.)

@reirab yeah, many of the same things apply, but now I'm thinking there's enough unique about the cockpit.

The cockpit forms part of the pressurised section of the fuselage and the window cutouts will weaken the fuselage structure. So the cockpit windows are so small for the same reason as the cabin windows are small. A great deal of design effort however goes into the cockpit design layout in order to ensure that the pilots have adequate visibility from the cockpit. For example the vertical and horizontal angles that a pilot must be able to look out of the cockpit with is governed by FAA or EASA regulations.

Indeed things could always be more... wings could be could be used to store baggage, radios could be stereo, landing lights could be brighter, windows could be opened, there could be a single pilot, or even none. planes could have built-in lifts, crews could have video links with ATC to see the ATC guys, and ATC guys could have a look within the cabin to be sure the crew is not sleeping. All that is possible, but comes at a price, and to the detriment of something else. Do pilots complain about cockpit windows in the first place?

@mins Indeed - everything is a trade-off of some description and that alone can pretty much answer almost any decision as to "Why isn't it like this instead?"

@Dan: I'd love to see questions like What are the factors determining or limiting the size of a commercial cockpit windshield.

The reason is that bigger windows really aren't necessary. They are plenty big for the crew to see the runway just fine. Pilots have more than 180 degrees of visibility, and plenty of vertical visibility as well. Traffic routes and patterns are designed to make it easy for the pilots to see the runway as well. If the runway is behind you, then you need to turn around. If it's below what you can see out the window, you are too high anyway. And if it's above what you can see out the window, you probably have more important issues than finding the runway.
Larger windows can help spot danger like other aircraft, but airliners are almost always flown under Instrument Flight Rules (IFR) where ATC is responsible for keeping them clear of conflicting traffic.
In the past, some airliners did have larger window area. This was more important in an era where instrument navigation was more limited, and the pilots relied much more on their view outside. It was also more common on aircraft with military heritage, where it was more likely for pilots to find themselves relying on visual references instead of navigational aids.
However, modern navigation systems have made larger windows even less useful. The instruments can tell pilots just about anything they need to know about anything they can't see out the windows. The 737 actually started with "eyebrow windows" above the main row of windows. Modern systems have made those less useful, and the extra holes in the fuselage can cause cracks over time. Most 737's delivered today don't have them, and some of the old planes even get those windows plugged.
Aside from the functional reasons for not increasing window size, the same structural concerns of cabin windows will apply here as well.

I read the brow windows on 737s get plugged because sun has a nasty habit of shining through them and making the instruments harder to see due to reflections. Structurally it won't help, because the skin already isn't one piece.

ROFL "And if it's above what you can see out the window, you probably have more important issues than finding the runway."

Re "airliners are almost always flown under Instrument Flight Rules (IFR) where ATC is responsible for keeping them clear of conflicting traffic." -- this statement is arguably ambiguous or incorrect. Only in certain airspace (Class C or higher) is ATC responsible for keeping IFR aircraft clear of VFR traffic-- and in fact in that very same airspace, ATC is also responsible for keeping VFR traffic clear of VFR traffic. So flying under IFR actually has nothing to do with it. Of course in Class A airspace, there is no VFR traffic, but airliner operations are not limited to Class A airspace.

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Why doesn't the Concorde have any wing fences?
A Wing Fence is a straight, vertical "fence" running from forwards to backwards down the wing.

This MiG-17 has three fences per wing. I've also seen them on delta-winged aircraft such as the MiG-25.
The purpose of the wing fence is to prevent the air from flowing sideways over the wing. Air instead becomes trapped between the fences and flows front to back as normal. This is the ideal condition for creating lift, because the air follows the camber of the wing.
So why doesn't the Concorde have them?

The even more extreme sweep (at first) of Concorde's wing seems to me like it would induce even more sideways-flowing air, meaning it should benefit even more from wing fences.
(I noticed the Tupolev Tu-144 doesn't have them either.)

At least in some cases the modern replacement is a vortilon. But it seems the more modern and sophisticated wing-designs doesn't need fences.  https://en.wikipedia.org/wiki/Vortilon

As I understand it, wing fences only really help when the aircraft is approaching stall speed. The slower airflow over the wing begins to move sideways causing an increase in stall speed and a nose-up tendency in a stall. I'm guessing that part of the solution used on the Concorde was to fly above stall speed high enough to not induce that issue.

Flying above stall speed is clever ^^

I am not sure I would even call MiG-25 wings a delta. The leading edge is not swept that much. Less than that of MiG-15/17/19 I think.

@JanHudec I've heard it described as a "cropped delta". In any case, it's a Mach 2+ craft with relatively high wing sweep, so I thought that it and the Concorde would both push air to the side more than front-to-back. One has wing-fences while the other doesn't, which is why I mentioned it.

@DrZ214, MiG-25 has wing sweep around 40°, way too low for the normal flow to remain subsonic at max speed (so the wing has to be, and is, supersonic) and way too low to generate vortex lift without leading edge extensions, which it does not have.

Wing fences were crude and simple way to do things back in 40's and 50's when we knew little about airflow in general and trans- and supersonic flow in particular. We've learned a lot since then so we have better ways since. Concorde's design was cutting-edge technology, we still don't use all of this tech even today.

Could you source your image? Or (if personal photo) tell us the location?

The main purpose of the wing fence is to prevent the boundary layer thickening in the wing due to the spanwise flow observed in swept wing, as can be seen below.

Image from fuckyeahfluiddynamics.tumblr.com
This is the reason for having wing fence in aircraft such as Mig-25- to avert stall and improve stability. In case of highly swept wing like the Concorde, this is not required as the mode of generation of lift is different- using vortex lift.

Vortices over the Concorde wing, Separation in three-dimensional steady flow, ONERA
In fact, there is a significant cross flow, as can be seen in the following image:

Flow above the Concorde Wing, Separation in three-dimensional steady flow, ONERA
Due to this, the wing fences are not required in case of highly swept wings like Delta and Ogival wings (as in Concorde), where separation leads to vortex lift. Mig- 25 wings, though highly swept, was neither of these- the wing generated lift by conventional manner.

Strange. Why are those "wingtip vorticies" over the middle of the wing instead of the tip?

@DrZ214 They are not the conventional wingtip vortices. The mechanism of lift generation in this case is different- it is through the vortices themselves. See the question on vortex lift for details.

@DrZ214: The vortex in delta wings (and indeed a square or circle wing) are different from wingtip vortex. Think of it this way: hold a large piece of squarish cardboard or foam vertically (like a wall, not a wing). Then push it forward. Obviously the area behind it will be a partial vacuum. Also obviously the vacuum cannot last, air will rush in to fill it - thus forming the vortex. It is this partial vacuum that is a source of lift, not the vortex. That's one reason why this sort of lift is called "vortex shedding" because the vortex gets created and destroyed continuously.

the boundary layer thickening in the wing due to the spanwise flow observed in swept wing What does this mean exactly? Are you talking about compressive heating against the leading edge? Also, in that first grayscale image, are the brighter spots representative of "thicker" flow?

I'm pretty naive to this concept, but it would seem like the fences are not just unnecessary, but would be harmful to the vortex lift effect by possibly decreasing the cross flow above the wing at the bottom of the vortex. Is that accurate?

The Concorde makes use of vortex lift (link).
While one of the reasons for applying wing fences is to prevent air from flowing sideways, they are also able to create vortices (R. Whitford, Design for Air Combat, 1987), though this only applies to wing fences that don't extend over the entire wing.
The need for vortex generators can be overcome if the wing itself is designed in such a way that a vortex is generated. Besides that, the vortex itself is very stable and also prevents air from flowing sideways.

(source)

authors: Азамат Хафизатдинов

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Should head stay level or inclined in a bank?
I just started to receive instructions to become a glider pilot, yay me! :)
I (and my instructor) have noticed that I have the tendency to keep the head level (eye-line parallel to horizon) in turns and my instructor says that wouldn't be a good idea, because at some point I won't be able to keep the head level anymore, so I should practice with keeping my head level to the lateral axis of the plane, i.e. inclined together with the bank and eye-line parallel with the wings.
What do professional or experienced glider pilots do? Is it maybe a matter of taste? Or should I let go of this tendency because at some point I just can't use horizon and gravity-sensation to orient myself (due to g-forces, spinning, etc.)?

Are you asking what professional glider pilots do? At least from a GA standpoint, when flying I don't tilt my head to match the horizon as I make turns. I need to be head outside to watch attitude and head inside to watch instruments, and that would make for a very sore neck and spatial disorientation very quickly. Have you ever seen what a Barany chair does? Seems like you would get the same effect...

Thanks for the comment. I clarified. What is GA? Are you saying that because I need to read off instrumentation constantly it makes more sense to keep the head level with those? That makes sense,hadn't thought of that,but I don't have a prob reading them at inclined angle.

GA means general aviation,  typically propeller driven private airplanes. Keeping my head straight also helps correlate the artificial horizon with the actual horizon but I understand that many gliders don't have that instrument.

I'd venture to guess that it's something that will go away naturally. You're probably just not used to being at an angle while seated. Once you get accustomed to looking from instruments to window like Ron said you'll probably stop doing it

FWIW, I wasn't taught to keep my neck straight, and initially I did what you do. I don't know when it disappeared, but as @TomMcW says, it kind of just came naturally after a while.

Very high bank angles aren't very rare in my gliding experience, you don't want to be turning your head 75 degrees from your body, also you need to be able to look into the turn for lookout.

I'm amazed how much interest this question created? I think this has become my highest ranked and most active question ever on stackexchange, and I'm for years on stackoverflow. ;)

I am not a glider pilot, but the principle holds for flying any fixed wing aircraft1: you should keep your head level with the aircraft, as you described, not the horizon.
As your instructor explained, there may come—or will come—a time when you are not able to keep your head level with the horizon due to the nature of a particular maneuver or orientation, and that is a practical matter to consider. It is certainly difficult to incline one's head 60° to the side in a steep turn.
From my perspective, however, the more important reason is that your frame of reference needs to be the aircraft. You are used to viewing the world with the ground as your frame of reference; down is down, up is up. As you learn to fly, however, you need to learn a new way to view three dimensional space, or—more specifically—the way you move through that space in the aircraft. It might sound cheesy, but you really need to learn to become one with the aircraft and hold the same perspective regardless of the aircraft's orientation.
Now, that being said, as you gain experience and develop your skills, you may come to realize that this perspective shift does not always require that your head remain rigidly perpendicular to your shoulders. Rather, it is a matter of perception which will eventually be independent of how your head is oriented relative to the aircraft. At my first flying job I spent much of my time in maneuvering flight and my head was rarely still—much less level with the aircraft. I spent a great deal of time with my head on a swivel, looking up and down, side to side, craning forward to clear into a turn, looking back into the cockpit, etc.
At this stage in your training, however, it is probably important to train to keep your head straight within the aircraft; certainly follow your instructor's instructions, and you should do well.
1This probably holds true for rotorcraft as well, or any aircraft for that matter. I am only sharing what I am familiar with.

I'm surprised to see that no one mentioned that waggling your head around in a moving aircraft will make you barf.

@HowardMiller I mention it in the very first comment to the OP's post above... That's what a Barany chair does...

@RonBeyer Sorry, I missed it. Sometime MEGO.

@HowardMiller Certain movements will tend towards motion sickness, but having one's head on a swivel, properly done need not induce nausea. For that matter, having your head "waggling" around by induced motion is common and—at least with experience—need not result in discomfort. I fly extensively in moderate turbulence and low level wind shear and only very rarely experience motion induced nausea.

@JonathanWalters When I fly commercial, I always ask for a seat over the wing root to minimize the effect of aircraft motion, and an aisle seat so I can get to the bathroom without climbing over people. In a small plane, though, I carry bonine. Once, I went sailing on a 32 foot sloop with several other people. I was fairly familiar with sailing because I had my own boat, but found that I was seasick for most of the sail, except when I was at the helm. Then I felt perfectly fine. I think when you have the controls, you can anticipate the motion better.

@HowardMiller Some people are more affected by motion; I used to be very prone motion sickness. In some circumstances having the controls can help, but externally induced motion is largely unrelated to having the controls.

I was taught (by multiple instructors) to keep head straight.  During my instrument training, I was told it helped w/spatial disorientation.  During my aerobatic training, my instructor sat behind me and would slap the back of my head if I did not keep it straight during maneuvers.
straight == not tilted, lean back and keep head firmly planted on headrest, do not compensate bank, etc.

could you precise what you mean by 'straight' please?

Straight relative to body and plane, gotcha. Thanks!

You need to keep your head level with reference to the airplane and not the horizon.  Until you learn this basic building block, other more advanced things will be difficult to grasp.
Airplane Flying Handbook, pg. 3-11

The pilot’s posture while seated in the airplane is very important, particularly during turns. It will affect the interpretation of outside visual references. At the beginning, the student may lean away from the turn in an attempt to remain upright in relation to the ground rather than ride with the airplane. This should be corrected immediately if the student is to properly learn to use visual references. [Figure 3-12]

Level with the wings.
If you keep level with the horizon then you are not maintaining an even lookout all round for a start and when you find yourself in a stack of other gliders in a thermal you need to keep a good lookout even with FLARM.
In addition what the head does the body tends to follow and so you may find yourself unintentionally decreasing the bank angle to match your head and then slipping out of the turn.

what's "FLARM"?

@Federico: A collision avoidance system for GA aircraft. See https://flarm.com for more.

As others have said, keep your head straight in reference to the aircraft.  To add to Ron's comment, head leaning can lead to coriolis illusion.

I find this an odd question.
When I am looking over the nose, my head is wings level.
But most of the time my head is moving around anyway, scanning for traffic and scanning instruments.
And especially when turning, I am looking through the turn which means say 30 degrees to the side of the nose, and up and down over the horizon.
So I can't say how my head is oriented, except to say it's pointed at where I want to look.

Exactly because of your pointing statement in the last sentence, it's perfectly possible to look over the nose with a head angle that is inclined with reference to the wings. So I don't see the oddity, and at least 14 others seem to find it an okay question, too. ;)

Who looks over the nose in a turn?

I misunderstood that you tried to use the nose example as the exception when you would be level with the wings. But because of the importance of pointing not of head position, one has the choice to look into the turn with the head level with the wings or otherwise. Because the looking of 30 degrees into the turn can be done with head yaw alone, without any head roll, and that seems what the FAA prefers as well.

@K.-MichaelAye Except in some (high wing) aircraft you may need to "roll" your head to look into a turn adequately, which is my case. But that is not necessarily normative, and certainly not for gliders.


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What is vortex lift?
In a recent question I asked, I was given an answer by Peter Kämpf and he described something about lift being created by a vortex used on delta wings and the Bird of Prey wing.
How exactly does vortex lift form to generate lift and on what other types of wing is it used on?

A picture would be helpful.

What does the WikiPedia article not give you?

@Simon Didn't know their was wiki page. I am confused about a normal wing and how air is not sucked downwards due to the large vortex on the wing tip.

@mins: aeroalias only added the pictures after Ethan asked this question.

@PeterKämpf no, aerolias' edit is from 11:17, this question has been posted at 11:19

@Federico: So you think that Ethan read the edited answer and typed in his question all within two minutes?

@PeterKämpf no, I think it is wrong to state that the edit has been done after this question got posted.

@Federico: Technically you are right. It only took Ethan some time, and he was not aware of the edit when he asked this question. Besides, the edit is not really helpful, and this question lacks a good answer here on Aviation SE.

Just to clarify, when I edited the answer, I had no idea that this question had been asked. I'm pretty sure Ethan had not seen my edited answer when he asked this question.

@PeterKämpf I am not debating the contents nor the question, I pointed out that your comment was wrong, everything else is your own doing.

@PeterKampf I did not see the edit my apologies. I fell alsleep right after the question has been asked.

The vortex lift is the method by which highly swept wings (like delta wings) produce lift at high angles of attack.
In the case of wings having sharp, highly swept leading edges like delta wings, the leading-edge separation vortex phenomenon occurs at subsonic speeds. However, the separation does not destroy the lift as in the case of low sweep wings; instead, it forms two vortices which are (nearly) parallel to the wing edges.

Vortices over a delta wing with a sweep angle of 70°, Separation in three-dimensional steady flow, ONERA
In case of Concorde, the vortices look like this:

Vortices over the Concorde wing, Separation in three-dimensional steady flow, ONERA
A cross-section of the vortices looks like this:

Flow above the Concorde Wing, Separation in three-dimensional steady flow, ONERA
Unlike the (bound) vortex of the conventional wing, these are real in the sense that they represent an actual mass of air rotating at high speed compared to the air in front of the wing.
The vortices form 'vortex sheets' along the wing. Air is sucked into the vortex sheets and accelerated downward. As the airspeed in the vortex is high, the pressure is low. This low pressure on the upper surface produces lift.
From Low Aspect Ratio Wings at High Angles of Attack at adg.stanford.edu:

...the leading edge vortices, increase the wing lift in a nonlinear manner. The vortex can be viewed as reducing the upper surface pressures by inducing higher velocities on the upper surface.

Source: adg.stanford.edu
Basically, the vortices increase the lift from the baseline (potential flow) values, with the extra force being equal to the loss of leading edge suction associated with the separated flow in high angle of attack highly swept wings. This analogy gives the same result as before.

A Concept of the Vortex Lift of Sharp-Edge Delta Wings based on a Leading-Edge suction Analogy, Edward C. Polhamus, NASA Technical note D-3767
A major advantage of vortex lift generation is that it is effective at high angles of attack, over which the wings would normally stall. Actually, that Concorde used this method of lift generation is the reason for the high AoA during landing and its droop down nose.

Source: http://soliton.ae.gatech.edu
One thing to be noted is that vortex lift generation happens at high angles of attack or in case the wings are designed to generate separated flow (with sharp edges as in Concorde) in normal conditions. Otherwise, the lift generation is by conventional means.
The same mechanism is responsible for lift generation in:

Strakes

By U.S. Navy photo by Mass Communication Specialist 2nd Class James R. Evans [Public domain], via Wikimedia Commons

Chines

Image from Effects of Vertical Tail and Inlet/Strake Geometry on Stability of a Diamond-Wing Fighter Configuration by Mitchell E. Fossum et al., accessed through http://enu.kz/

Canards

Source: forum.keypublishing.com

So the lift on the average airliner doesn't use the vortex as an advantage but generates lift in the normal way while delta wings use the vortex to generate lift.

Yes. One main condition for vortex lift generation is high sweepback angles. The 'average' airliner has a wing sweep of ~30 degrees, while the vortex lift mechanism comes into play at sweep angle is around twice that value. Also, they rarely fly at high angles of attack (like delta wings).

What aircraft is that in the photo below "strakes"?

@DrZ214 That is an F/A-18, the image is also part of the wikipedia article about strakes

What it is
Vortex lift needs two conditions:

High leading edge sweep (ideally 60° and above)
High angle of attack, the more sweep, the smaller the angle when vortex lift becomes effective.

Vortex lift is caused by flow separation at the leading edge. While this indicates a severe stall on unswept wings, the separated flow along a highly swept edge will roll up and will produce a conical, stable vortex. The speeds due to this helical movement of air cause air pressure over the wing to drop, which provides the suction that produces the lift.
If the angle of attack is increased further, the vortex becomes unstable and bursts. If this happens while it is still over the wing, the aircraft becomes unstable - the bursting of the vortex is a highly dynamic process, leading to buffet and an unstable, self-reinforcing rolling motion.
Where else is vortex lift used?
Vortex lift is helpful when lift is needed at high angles of attack or sideslip. Besides on delta wings, it is used in:

Strakes or leading edge extensions ahead of the wing
Strakes or fillets ahead of the vertical tail surface
highly swept canards or fins

Pretty pictures

Picture of CFD simulations on the F-18, taken from this Aerospaceweb page.
The grey ribbons show how the flow curls up past the leading edge of the strake and stays close to the upper surface of the wing.

Flow visualisation on a delta wing (picture source). This picture was taken in a water tunnel, and dyes were injected at the leading edge to make the streamlines visible.


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What is this delta wing aircraft parked at NASA Ames Research Center at Moffett Field?
What is the aircraft shown at these coordinates? https://goo.gl/maps/VTc153EacEL2
This is at NASA Ames Research Center at Moffett Field. Per the Ames map it is located north behind building N-127 (Warehouse) and west of the VTOL pad.
It has a dome canopy and a delta wing configuration. It is approximately the size of a fighter aircraft. A very blurry picture of the tail is visible by using Google Streetview from the Moffett golf course and looking west across runway 41R.

Delta-wing aircraft tend to look pretty similar from top-down; do you have any other photos of it from another angle?  That might help identify it, either by showing aircraft markings, and/or a better view from the side to pick out more features.

@RalphJ - No. Just another one of those things found while goofing off on Google Earth. Google 3D does somewhat show a side profile but I question its accuracy.

The F-106 seems like a close match and there is one listed in the NASA inventory at https://en.wikipedia.org/wiki/List_of_NASA_aircraft

Could it be a B-58?

@MikeBrass No, the B-58 wing shape is different + the underwing engines are very obvious from a top-down view.

The challenge here is that its clearly parked in a boneyard. Engines could be off. It's also NASA boneyard, it seems possible that it's a standard platform with modifications.

Bonus question:  what is the plane beside it?

@Freiheit  Definitely not a F-106.  The wing on the mystery aircraft is taller than it is wide and the f-106 is almost exactly as tall as it is wide.  The wind tips are also flattened in your picture as opposed to the F-106 but I suppose a saw could have fixed it that way...

I don't agree with the assessment that the F-106 is similar. The wingtips are pointy for the 106, rounded for the one in the picture. Given the surrounding scraps, it could be a mock-up, rather than an actual aircraft.

Looking at the 3D view in Google maps it very much looks like a F5D with a modified wing. However there were only 4 F5D's built of which two went to NASA. Both are now in musea. The F5D was flow with modified wings by NASA, in a research programme for supersonic transport. Perhaps it is a windtunnel model for studies that we are looking at?

If the pinned aircraft "is the size of a fighter", then the one next to it must be the size of a lawn dart!

It is a STOVL E-7 Model.. It is not an actual aircraft.
https://artsandculture.google.com/asset/e-7-stovl-fighter-model-testing-in-ames-40x80ft-wind-tunnel-tom-trower/RQEHhSh_zF3TuQ?ms=%7B%22x%22%3A0.5%2C%22y%22%3A0.5%2C%22z%22%3A9.425586796493714%2C%22size%22%3A%7B%22width%22%3A1.2166568742655705%2C%22height%22%3A1.2375000000000007%7D%7D

The enlarged photo shows the inlet duct covers. It looks like they left the lifting cables just laying on top. This is a model that never flew.

It would do a lot to include some text and/or an image from your link. That link could go dead in an internet minute leaving your answer useless. Please feel free to [edit] something in to bring a little more substance.

I'm not convinced-- the wind tunnel model appears to have sharper front and back corners on the wingtips than the photo in the original post.

@quietflyer Google messes up photos when they try to create 3D. If you look at just the white wing tips on enlargement, they match the model pretty closely.

This photo would indicate that the aircraft in question is the same size as the Wind Tunnel model.

@Pilothead -- ok I see that now..

I think @DeltaLima nailed it in comments. I'd happily accept an answer if he/she would care to post one.
From: https://www.nasa.gov/centers/dryden/multimedia/imagegallery/F-5D/F-5D_proj_desc.html

After the Dyna-Soar program was canceled in December 1963 the F5D-1
  (708) went to Ames Research Center, installed with a ogee wing for an
  evaluation for Concorde Supersonic Transport wing study. The F5D-1
  (802) stayed on at NASA Flight Research Center contributing to various
  tests. It became a flight simulator for the M2-F2, and a chase for the
  lifting bodies until 1970. In May 1970 the Douglas F5D-1 Skylancer
  (NASA 802) was retired and donated to the Neil A. Armstrong Museum in
  Wapakoneta, Ohio, to rest beside the light plane in which Armstrong
  learned to fly.

This shows that an airframe was at Ames and it had a modified wing. I think it suggests that the airframe never left Ames.

I found that same text and researched where 708 and 802 are now. Both are, according to the NASA links, in a museum. So it must be something else.

This is not an F5. The wingtip is much more round, and the modification made by NASA made the wing look like half an infinity symbol, not straight as in your Google photo.  https://youtu.be/zRYZPaHY7rU?t=22


text:
What are the lettered and numbered signs next to airport runways and taxiways?

What do these numbers signify?
Can pilots see them while landing?
Do they matter while taking off?
Do Control Tower mention these while guiding planes on runway?

These are not on the runway instead they are on the taxi way between the photographer and the plane.

https://en.wikipedia.org/wiki/Taxiway, most of the information is available here.

runway numbers for the runways.

think of them as street names

From left to right in your image

16 CAT II / III

Refers to the runway number, and also the ILS Category that particular runway supports. However that is just secondary information. The primary purpose and position of this sign is actually to indicate the hold-short position for the numbered runway.

E6 34/16

Refers to the taxiway designation (E6) and the runway(s) it leads to (34/16 being the same runway 34 one way and 16 the other)

{Unreadable} and E6 L→ 10→

Both of these signs are the same, one is pretty much unreadable the other is again on taxiway E6 and points that Taxiway Lima (L) is to the right as is runway 10.

Pilots are unable to see these signs on landing, signs like this on taxiways are almost exclusively for ground operations - not just the aircraft but the variety of vehicles that operate on the apron of an airfield.

These signs are equally not useful at the point of takeoff, none of the information presented on them is relevant for that phase of operation.

Pilots will have a "map" of an airfield with them which will indicate the same information as presented on these signs. Ground controllers will typically mention which taxiways to use when guiding aircraft (and other vehicles) using the apron and taxiways. For example, at the airfield pictured the ground controllers may have instructed the pilot that they were cleared to "hold short of Runway 10 via Taxiway Echo 6" or may have told a landing aircraft that they should "clear the active runway at Taxiway Lima".
Here is a graphic that many websites use a variation of:

Source: americanflyers.net

The "16 CAT II/III" does NOT tell you what categories of ILS the runway offers (though that can be inferred in those cases where such signs are present). They are hold lines - see the 4th red box in your graphic. Plenty of Cat III runways do not have such signs saying CAT III on them - they may have just an ILS hold line or sometimes the runway hold line is located such that no separate ILS hold line is necessary. You need other documents to know for sure what categories of ILS a given runway supports.

@RalphJ thanks - although I knew that they were actually the hold short position to stop  interference with the precision ILS transmitter I failed to include that in my answer. Thanks for the pointer.

Yes they signify taxiways (yellow and black markings) and runways (red and white markings)
While landing no, you are too busy to look at taxiway/runway markings.
Sure they do matter while taking off. Or to be more specific, they matter for taxi and a safe takeoff. Check out what happened to Singapore Airlines flight 006 for not watching the runway markings and taking off from wrong runway.
When ATC clears a plane for taxi, they mention the taxiways the crew should follow. Though the airport charts have the taxiways marked, they should keep an eye out to make sure they didn't take the wrong turn.

Damn that bad weather! +1 for providing the link. Thanks.


text:
What would be a minimum ICAO Aerodrome Reference Code for C-130?
I was wondering, mostly of military planes are not mentioned by ICAO on its papers (for sure, it is dedicated to civil aviation), but it's common to see military cargo planes operating on Civil Airports in Civil Aprons. I believe that mostly of this military cargo planes are intended to Takeoff/Land with higher loads in a significant short distance with low ACN number (in comparison with civil cargo aircraft).
So, if we consider a C-130H "Hercules" operating in a Civil Airport, what would be a minimum ICAO Aerodrome Reference Code (like 4D, 3D, 2D?
I believe Code Element 2 is a "D" due the wingspan (around 40m) and Outer main gear wheel span of 4.34m.

Reference codes you mention are introduced by annex 14 and can be found in Doc 9981 (PANS-ADR).

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Radius of Turn Query

I am making a project related to flight paths. If we are given two points in space (lat,long) and the flight headings(angles) that the flight should attain at that points.
By using the radius of turn formula, I checked whether the flight can reach the next point or not, but how can I check whether the flight can reach both the points with the specified heading or not??
Please help me out.. Thanks in advance...
Sorry for the ambiguity in the question. In the figure attached, if I need to go through points 1, 2 and 3 so on which are few points in the flight path. And if I am given (lat,long,heading the flight should attain at that point) for each point, I need to find out whether points can be reached or not. Also please suggest me the solution if I need the same heading at all the three points (say x).
And the flight should not take more than 1 turn i.e. loops are not allowed (flight should always move towards the destination).

Now, i need to head from source to destination along points 1,2,3 with same heading(q) at these three points. For example take q as the angle between source and destination. Now i want to move towards destination i.e by not turning back and no loops. Just want know whether its feasible for flight with specific radius of turn to reach the next point or not.
Or what should the minimum vertical distance and horizontal distance between the points be in order to satisfy both the points.

But we can't determine the minimum radius exactly right?? In the figure 3, point 1 can be reached with the specified heading with distance below 2 * rmin. So the condition be like delta(x) > rmin definitely because it can't reach close neighboring points but what should be next condition to check whether it can reach with required heading or not. If it is like delta(x) > 2 * rmin then figure 3 would contradict it, right?

If you can check one, you can check next one after that in the same way, no?

Tags are quite off and the title is also non-descriptive. Also, you can always reach another location, only question is how many maneuvers you have to do in order to obtain said result.

This sounds more like a geometry puzzle than an aviation question. If we do not know the start and end points, how should we help with the optimum route, given that you need to avoid turns? I feel like the expert in this video.

@amarender: Please state your problem with all boundary conditions. I expect that you will help yourself the most when you have clearly expressed what exactly you want to know.

@amarender: please note that in your second figure you have 2 turns between each pair of points.

@Federico Ya even though it has 2 turns , its always moving towards the destination which is my requirement. And also it can have more turns but not take loops or reverse directions ..

what should the minimum vertical distance and horizontal distance between the points be in order to satisfy both the points I strongly suggest you to study a bit of euclidean geometry and basic flight kinematics.

The difference in headings gives you the number of degrees you need to turn. The distance between the two points then becomes a secant to the turning circle which should give you the turn radius by applying some basic geometry. I assume that the turn starts on point 1 and ends on reaching point 2. Like this:

Now you first need to calculate the turn radius, using the equation for secant length: Start with the difference between the two headings and call that angle $\varphi$. Then the difference between two points $s$ is $$s = 2\cdot r \cdot sin\frac{\varphi}{2}$$
Since you know $s$, you can calculate the turn radius $r$ by a simple transformation: $$r = \frac{s}{2\cdot sin \frac{\varphi}{2}}$$
Now you need to check whether the airplane will be able to turn with that radius. This depends on wing loading $\frac{m}{S}$, density of air $\rho$ and maximum lift coefficient $c_{L_{max}}$, and the equation for the minimum turn radius is $$r_{min} = \frac{2\cdot m}{c_{L_{max}}\cdot S \cdot \rho}$$
Compare both values of $r$ and check if $r > r_{min}$. Then the turn is physically possible.
Now I only hope that this is really what you asked. Your question leaves a lot of room for interpretation.

EDIT: @amarender has updated the question. Now the heading is identical at all intermediate points, and below is one of many possible solutions (blue line):

I indicated with pie segments where the turns are; of course, the one single turn condition has to be interpreted as one turn per intermediate point. The sections between the turns are straight, and all bring the aircraft closer to the destination if the heading at the intermediate points is identical to the heading from target to destination.
When do we have a condition where a solution is not possible? Only if the points are too close together to satisfy the condition of the minimum turning radius. By rotating the coordinate system such that the desired heading at the points is parallel to the x axis, this is easier to show:

The distance between two points in the direction of the desired heading $\Delta x$ must not be smaller than the diameter of the minimum possible circle ($2\cdot r_{min}$) using the equation above. The green line above is for $\Delta x > 2\cdot r_{min}$, and the red line for the boundary condition $\Delta x = 2\cdot r_{min}$. To be precise, the equation above does not put a limit on the maximum load factor $n_{z_{max}}$ - if this needs to be considered, the minimum radius is:
$$r_{min} = \frac{2\cdot \frac{n_{z_{max}}}{\sqrt{n^2_{z_{max}}-1}}}{c_{L_{max}}\cdot \frac{S}{m} \cdot \rho}$$
Things get more complicated when the heading at the points is different from the heading from the source to the destination. Then $\Delta y$ becomes important as well, and when it gets large relatively to $\Delta x$, the straight connection between turns will point away from the target very soon.

this is actually only one of 12 possible situations (assuming same altitude for both points). As I said in my comment to the question, it is always possible to reach both the points with the specified heading, this answer checks only if this is possible performing only one turn.

@Federico: You are right, I took a guess what amarender might mean. But flying an arbitrary path between the two points is trivial, so I took the case with the shortest circular connection between both.

I understand that, I took the liberty of underline this since you did not explicitely stated the assumption in your answer. Also, there are optimal solutions also for when $r<r_{min}$, but they involve multiple turns.

I'd bet that the OP asks for reaching both points in one single turn where the turn is less than 180 degrees, and the plane is turning a constant rate all the time between the two points. Otherwise, the turn is always possible by overshooting then turning in the opposite direction. If more than one turn is allows then the question is moot, there're infinite number of solutions.

What if i need the same heading angle at all the points and flight cannot take more than one turn i.e if it heads only towards destination all the time.Flight also can't take loops. Imagine only one source and one destination and points 1,2,3 lie between them.

@PeterKämpf : Ya the flight can't attain the points with same heading in the figure which you drew. Could you explain the solution for the problem, if the flight has to move from source to destination as specified in my figure.

@PeterKämpf : Sorry if i am wrong but the case in figure 3 would contradict your answer of delta(x) being greater than 2 * rmin. If yes, what would be the minimum delta(x) in such a case.

@amarender: I don't get it. The red circles are for a minimum turnig radius equal to $\frac{\Delta x}{2}$, and the green circles for a smaller $r_{min}$. Where is the contradiction?

this question is becoming more and more about homework and is already extremely localized. Your answer already gives all the details needed, I would refrain from continuing.

@PeterKämpf : If the flight turns at the point on the green circle (intersection of green and black circle) then point 1 can be reached with the required heading, right? Then, the delta(x) is less then 2 times the minimum radius. Even the delta(x) is less than 2rmin(can be visualized from figure) the flight will be able to reach the point 1. But we came to conclusion that delta(x) must be at least greater than 2rmin. This is what i am concerned from the time i asked the question. May be, i was not clear in asking the question, which led to confusion, Apologies for that!!

@amarender: Yes, the above is only for $\Delta y > 2\cdot r_{min}$. But now that you know $r_{min}$, the rest is basic geometry. This forum is not for solving your geometry problems.


text:
Does the validation of an ICAO licence allow you to fly internationally?
Hypothetical situation:
A pilot with an ICAO-compliant PPL licence from Australia goes to France and France issues a licence validation in accordance with the Chicago Convention, thus allowing him to fly aircraft registered in France.
If this pilot wants to take a French-registered aircraft to another country is this possible? From Article 33 of the chicago convention it seems like this should be allowed, but is it actually in practice?

Article 33
Recognition of certificates and licenses
Certificates of airworthiness and certificates of competency
and licenses issued or rendered valid by the contracting State
in which the aircraft is registered, shall be recognized as valid
by the other contracting States, provided that the requirements
under which such certificates or licences were issued or
rendered valid are equal to or above the minimum standards
which may be established from time to time pursuant to this
Convention.

Closely related

When doing the validation e.g. in France you will get a validation from an EASA member which enables you to fly an EASA registered plane world-wide.
EASA includes registered planes from
Austria
Belgium
Bulgaria
Croatia
Cyprus
Czech Republic
Denmark
Estonia
Finland
France
Germany
Greece
Hungary
Iceland
Ireland
Italy
Latvia
Liechtenstein
Lithuania
Luxembourg
Malta
Netherlands
Norway
Poland
Portugal
Romania
Slovakia
Slovenia
Spain
Sweden
Switzerland
United Kingdom


text:
Why will reducing the boarding time reduce the costs?
There are quite a number of publications about airplane boarding which all usually begin with something like:

Boarding is an expensive process that costs so much, that by reducing the boarding time airlines could save a lot of money.

However, none of the publications I've read says explicitly why the costs are reduced. Of course there is at least one obvious reason:

A shorter ground time reduces delays and thus the number of missed connecting flights, leading to fewer liabilities.

Further there is the possibility that a plane can do an additional flight during the day. But if a plane does 5 flights a day and saves 5 minutes per boarding, then the resulting 25 minutes would not suffice (at least in countries with start restrictions over the night).
So therefore the question: What factors play a role in saving money by reducing boarding time?

It should also probably be noted that quicker boarding doesn't necessarily mean faster turn-around times. Faster boarding won't get the bags loaded/unloaded any faster, the fuel loaded in faster, the pre-flight inspections performed any faster, etc. Faster boarding only decreases turn-around time when they're actually waiting on the last passenger to get on to depart (which, at least in my experience, isn't very often, though it does happen sometimes.)

According to a study by Boeing, boarding time is the biggest contributor to turn around time. After 30 mins at the gate/stand Heathrow starts charging widebody aircraft £210 an hour. After 24 hours, if you don't vacate when asked, the rate goes up eightfold.

@RedGrittyBrick how much does it charge for the first half hour?

@trengot: no parking charge for first 30 mins

It's not as much boarding as much as standing around does not generate any revenue.

Use Your Capital Assets: 25 minutes a day is a lot, and in some cases is equivalent to a domestic leg. Rather than arriving at 23:00, you could arrive at 22:30 and still squeeze in an additional short flight before midnight. If you're flying 10 flights a day and you can now fly 11, you're doing 10% more work with the same amount of assets. Estimates for a flight hour of a B737 are ≅ \$3000-\$3500 in the US, for an aircraft worth perhaps $60 million.
Fixed Costs: You're looking at a bit of fixed costs, such as insurance. The more flying time you can squeeze out of the plane, the cheaper each hour will become on average.
Less Staff, More Product: You need a certain number of ground staff. If you can cut the time required for an aircraft to be handled by ground staff by 20% (50 → 40 minutes), that potentially means that they could handle an additional aircraft in their shift. This principle applies for airline flight crew, as well. It's a lot cheaper to let the your pilot fly a few hours more than needing to hire another one.
Standing around is not always cheap either: Depending on airport cost/fee structure, you may be charged less for the ground time. This is not only standing around; for instance, Munich airport charges you €44 for every 30 minutes of using a ground power unit.

As well as the plane not going anywhere, sitting on the tarmac waiting for the squishy cargo to board eats up the crew's on-duty time.

@DavidRicherby in lack of proper knowledge I left that point out, since some airlines go for duty time by chocks on/off the wheels at the stand for instance, and I'm not sure how ground stops are counted.

Oh, OK. I wasn't even aware of that variation so good call on leaving it out. :-)

@MikeFoxtrot duty time is different than pay time and encompasses all of the time from your show time until the end of your day and has hard regulatory limits and often stricter limits mandated by CBA.

Finishing boarding of a particular flight on a particular day faster than average won't help much to reduce costs, as the airline has to compute flight schedules weeks or months in advance, so they have to calculate average boarding times for a particular aircraft, and factor that into schedules.
If the airlines could bring the average boarding time down, they could fit more flights, tighten up the timing of legs and itineraries, and lower the asset-hour per passenger-hour ratio, which would result in aggregate cost savings in gate costs, personnel costs, fuel, insurance, and financing costs.


text:
Why do high altitude air-launch plane designers choose to use two fuselages?
We see double fuselages in both the huge Stratolaunch vehicle and in Virgin Galactics high altitude launch vehicle. What about this design makes it desirable?

I edited the title as you seem to be asking about air-launch planes. If this is incorrect feel free to edit it back. There are lots of high altitude mono fuselage aircraft.

@Dave - Yes, you are correct. Thanks for the clarification.

It is simple to maintain symmetry if the launch vehicle is in the middle, with two fuselages on either side.
With only one fuselage, the departure of the launch vehicle would change the symmetry, and therefore the lateral stability.
One possible exception is if the launch vehicle is directly above or below a single fuselage.  This was done with the M-21 carrying a D-21 launch vehicle, where the D-21 was directly above the centerline of the M-21.

I think there is a also a height advantage having it slung below a wing vs being slung under a standard cabin. With an engine/tail assembly then on both sides of the payload, that would help with takeoff stability, giving the benefit of 2 engines without the need of a 4-engine plane and being slung under 1 wing only.

@CrossRoads: Think about the Space Shuttle and its modified 747 carrier, which was used to "launch" the Shuttle for drop tests.  Now think about how you'd put a Shuttle and its fuel tank on that single fuselage.

I think that was more a matter of what was available to carry something that big & heavy, vs customizing for the state of the art now. What's less expensive to get up in the air - 4 engines on one of worlds largest aircraft (maybe the largest at the time) or two engines on a custom body designed from the ground up as a launcher?

I envision the Stratolaunch being able to carry payloads that end up looking like the cabin section on this high wing twin, see 1:22 into the video https://www.youtube.com/watch?v=u_YTWgCuORc


text:
What is the function of ECS in airplane?
What is the function of ECS in airplane?

Welcome to aviation.SE! Your question title says ESC, but the body says ECS: which one is correct? It would also help if you can tell us which type of aircraft, and what the abbreviation stands for (if you know).

Related: Why is air mixed with bypass air on the A/C of an aircraft? with answers explaining how it works.

Do you mean Environmental Control System? If so, can you please edit to say that instead of just using the acronym? There are so many acronyms in aviation that a lot of them can mean different things in different contexts, especially if they're only 2 or 3 letters.

Welcome to aviation.SE.  The core of your question is made to expand and explain its title. You should expand it to provide further explanation of what you are looking for, possibly including  your current state of  understanding so that an appropriate level of explanation can be provided in answers.

Possible duplicate of Why is air mixed with bypass air on the A/C of an aircraft?

The ECS system is the Environmental Control System.  It is the system that handles pressurization and thermal control of the aircraft. The FAA has a nice handbook on it here. There is some more information on how it works on the new 747-8 here that may be of interest.


text:
Does it make sense towing airplanes to the head of airstrip by external (eg electric) means?
Does it make sense towing airplanes to the head of airstrip by electric means, whether internal or external?
Taxiing from the gate to the runway on jet power seems like a waste of fuel. Is it possible and it would make sense to tow the airplane to the start of take-off run by a motorized towing unit or perhaps with electric motors in the airplane's landing gear? Was this ever tested or assessed?

http://www.greentaxiing.com/, http://www.safranmbd.com/systems-equipment-178/electric-green-taxiing-system/, and many others.

Possible duplicate of How does an aircraft taxi?.  Not an exact duplicate but the answers to that question already cover pretty much everything that could be said here.

From the amount of fuel saved on the ground you must subtract the amount of fuel wasted by carrying those motors in flight. Such system should be most beneficial on short routes, while on longest ones it could possibly turn out wasting even more fuel.

This could be assesed, I'd say. The added fuel consumption in flights from weight of electric motors on airplane wheels, and sustract it form the fuel savings in not taxiing with turbines or other ICEs, but with specially devoted auxiliary means. You may like patents US2305237, JA Carpenter; US2333101 R E Mitton; US2312159, A Gulotta. Blessings +

Was [towing unit or motors in landing gear] ever tested or assessed?

Yes.
2014 Main Gear

Using the Auxiliary Power Unit (APU) generator to power motors on the main wheels, the EGTS taxiing system allows aircraft to push back without a tug and then taxi without requiring the use of the main aircraft engines. One wheel on each main gear is equipped with an electric motor to drive the aircraft

From EGTS

Airbus signs MoU with Honeywell and Safran to develop electric taxiing solution for the A320 Family

From Airbus press release

Lufthansa Technik has proven with a technology demonstrator that aircraft can be driven on ground by electric motors.

From Lufthansa Technik (with video)

2015 Towing Unit

As of 2015 Taxibot, a semi-robotic towbarless tractor which meets and connects to aircraft, is the only alternative taxiing system certified and currently in use by airlines in the market.

From Wikipedia

2016 Nose Gear

Honeywell and Safran have pulled the plug on their jointly conceived electric taxiing system, while rival developer WheelTug aims to enter service with its system in 2018.
"After thorough market analysis", the US supplier says, the two partners "agreed to stop work on the Electric Green Taxiing System due to dramatically lower oil prices and the current aviation industry's economic environment".
EGTS had been scheduled to enter service this year, with Honeywell and Safran having planned to produce a system for line- and retrofit on A320-family and Boeing 737 jets.
Meanwhile, Gibraltar-headquartered WheelTug plans to enter service with its nose gear-mounted electric drive system in 2018.
... initially on 737s and, at a later stage, on A320-family ...
Neither Airbus nor Boeing are supporting the programme

From FlightGlobal

Variations on this kind of scheme and designs are currently being worked on. From what I remember, using electric motors are still pretty expensive, and also you're using them for a very short time frame, such that you're lugging around extra weight that is useful for a tiny portion of time(aka more fuel burn). Tugs of various sorts have been used at various places as well.
Also consider that various engines need time to warm up before applying full throttle, sometimes just a few seconds, others a few minutes.

Opinion about wether electrical motors are relevant or not highly depend on who is givin it : A pilot looking at the fuel gauge after wasting one hour taxiing in a traffic jam will tell you "this is the solution..!" Same debate as the Winglets on 767s but with a slight difference : The extra weight of the winglets is worth it if your 767 is doing long haul flights. The extra weight of the electrical taxi is worth it if your A320 spend most of its time struggling in traffic jam on the ground along the year... :/

It's not just the weight of the electric motors, it's also the extra weight of batteries / fuel cells needed to power the motors. This makes a system like this worthwhile primarily for planes which fly very short routes and have to taxi long distances, while it's likely less efficient for other operating cases.

electric motors may also avoid the delays with waiting for tags etc

@Lightsider: Airliners already have pretty hefty batteries on board, so there might not be quite as much weight penalty as you assume.

@Lightsider It's my understanding that the electric motors are run using power from the APU, which is going to be running anyway during taxi to provide basic electrical power and ventilation to the aircraft. So there's no extra weight for the power source at all; the only extra weight is from the motors themselves and the system to distribute power to and control them.

This is an older question, and I have no answer, just experience:
When  I flew from Frankfurt (FRA) to Geneve (GVA) in January 2015, we were towed by a tug nearly all the taxiway to the runway. We turned to the parallel taxiway "N north" to get rid of the tug, then went back on taxiway "N" to the runway.
Here is our way as tracked by my mobile GPS:

The pilot told us in advance that we will to this, and that it is some kind of experiment / project / study about if it works. (I guess more from logistics / operation side)
From my data, I also have some time information:
12:12: End of push-back
12:15: Leaving terminal
12:21: turn right on "N North"
12:22: get rid of tug, start engines
12:27: Start moving again, and back to  "N"
12:29: arriving at runway
12:31: accelerating
If I remember correctly, the engines were started when we were on "N north", so 5-6 minutes before takeoff. However, we were taxiing to that place 20 without engines.
It would be interesting to know how much fuel could be saved by this procedure (It was a A320 or similar, I don't know), but at least, the aircraft didn't need any extra equipment.
(And yes, it was an ordinary tug, not this electrical stuff or similar.)

A few years back, the last semester of my MBA I took a class in advanced simulation and optimization, and the course professor told us about an industry-sponsored (Delta Airlines was the corporate sponsor) project the previous the class had worked on. They used simulation to explore the feasibility of using so-called "supertugs" to  move aircraft between the terminal ramps and the maintenance hangar at ATL. Aggregating all the scenarios they explored, the estimated project NPV was pretty solid (don't remember exact figures), so there are savings to be had, under the right circumstances.

@habu What's "NPV"?

@DavidRicherby Net present value, a measurement of the profitability of something (has not necessarily to do with aviation).

Along with fitting electric motors to allow push-back without a tug and more efficient taxiing, there is a plan to use tugs controlled by the aircraft for the taxi.

One simpler alternative for multi-engine aircraft is to taxi out with only some (or just one) of the engines running, starting the remainder within a few minutes of take-off. It might not save as much fuel as, say, a tug, but it can be done with existing aircraft and without any extra equipment.
The major drawback of any of these approaches is that running time during taxi contributes to check-out of the fuel, fuel system and other engine systems which all have to perform properly for a safe take off. Delaying engine startup reduces/limits the opportunity for such checks.
Nevertheless, I think this practice has been done/is being done in some situations.

Delta Air Lines experimented with a variation of this - taxiing on only the left engine. It did indeed save fuel, but maintenance costs on the right engine went through the roof. Turns out the nice, long, low idle "warm up" of a taxi is good for the engine. They went back to dual engine taxi and right engine maintenance levelled back out to match the left. – ScottSEA

Someone edited the question and my comments under the claim of 'improving readability'. Sorry, you can't substitute electrical where the original wording was 'external', it may be good, but it's different. Of course, you can propose changes in MY wording, but the polite and practical way for doing this is a copy and paste of MY text, and then, in response box, make your proposals for 'corrections'. How did someone manage to edit something that I can only access after entering an username and a keyword? This isn't good! Thanks, regards. Salut +

This has a tangential connection to the issue of electric towing for Aircraft,  patents: US2305237, J A Carpenter; US2312159, A Gulotta; US2333191, R E Mitton, all are about devices for having the undercarriage wheels rotating at touchdown, to avoid vibrations and tire problems.

Recently, I took a flight from Madrid, Spain, to Dusseldorf, Germany. The airplane was running in the Madrid land for 16 minutes before staring acceleration to takeoff, and just 6 min on arrival to Germany. How this influences fuel use and turbine life is unkonwn to me, same as the actual advantages of land running by electric engines in wheels until the time to warm up engines arrives. Salut +

authors: Сергей Алендарь


text:
When, and how to calculate this, you can expect too much Wing Dihedral generates Dutch Roll, increases in Drag?
The question comes from the Arup designs, and the engineer who turned it into blueprints, Hoffmann, ('A Novel All Wing Airplane', Raoul J Hoffman, Popular Aviation, March 1935, pp 163 and 196), they said adding dihedral would cause Dutch Roll and worsen lift to drag ratio. In low wing machines, dihedral could reduce 'Ground effect', sometimes undesirable. Blessings +

You seem to be asking a question about towing a trailer on the road -- rather than one specifically about aviation.  If that's incorrect, you should [edit] your question to make it clear what you're asking about.

The trailer yawing, and rolling a little bit due to spring compression, behind a car is nothing like Dutch Roll, which is an oscillation about all 3 axes and all 3 motions feeding each other. Not a good analogy.

@JohnK the Dutch roll is normally described as not containing pitch oscillations, just roll and yaw

It's a yaw/roll coupling oscillation and energy system, but there are also pitch movements with the nose rising and falling somewhat during the roll/yaw motion.

Voting to keep this open. The basic question is very relevant to this site: what are the design parameters for dutch roll stability in aeroplanes.

It is a shame some folks focus on "dihedral" as the source of Dutch Roll when there is that huge tail sticking up in the back.  Doesn't it make one wonder what a change in relative wind does to rolling forces there. Dihedral gets blamed when there are other design fixes for the issue.  Vertical CG must be considered.  There are ways of reducing "dihedral effect" without reducing dihedral.  Oh, and please reference Hoffman's blueprints.

The question was about Dihedral effect on Aircraft's Dutch Roll, and calculations about this, the mention of the Car towing an small trailer was there just to point something similar to Dutch Roll can happen also in road vehicles, but question is about Aviation, as only Airplanes have 'Dihedral'. Would you please reconsider posting it? Thanks Blessings +

Dutch Roll is a 2-dimenional dynamic stability issue in aeroplanes. A little  trolley towed by a car can be used to illustrate what happens, but the small amount of roll is purely an effect of directional instability, it is a one-dimensional issue.

The pic above shows a damped dutch roll. The graph shows the two coupled variables, roll rate and yaw rate, and an unstable dutch roll occurs when the time delay between the peaks is such that they amplify each other.

The dutch roll is not the only multi-dimensional asymmetrical stability issue to be considered, the other one being the spiralling descent. Both the dutch roll and the spiral descent motions need to experience positive damping.

For positive damping of the spiral movement, stability matrix E must be > 0, with $$E = C_L(C_{l,\beta} \cdot C_{n,r} - C_{n,\beta} \cdot C_{l,r})$$
For positive damping of the dutch roll, the Routh-Hurwitz discriminant R must be > 0, with R being a conglomeration of partial linear equations such as equation E above.

So both E and R must be >0, and both are determined to a large extent by:

dihedral coefficient $C_{l,\beta}$, larger for larger dihedral angle
static directional stability $C_{n,\beta}$, larger with larger tail volume

But changing the tail volume also changes other stability parameters, such as $C_{n,r}$ and $C_{Y,\beta}$. Effects of varying $C_{l,\beta}$ and $C_{n,\beta}$ on E and R are represented in the side stability diagram above, showing the area where both E > 0 and R > 0.
From the diagram it can be seen that when dihedral $C_{l,\beta}$ is increased, $C_{n,\beta}$ must be increased as well. A larger tail volume achieves this, but requires further analysis on the remaining a-symmetrical dynamic equations.


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Does it make sense having a MiG 19 fuselage 'area ruled'?
If the MiG 19 fuselage diameters followed the Area Rule, instead of being a constant size cigar, perhaps transonic performance will be improved; someone answered to this question elsewhere indicating the airplane is already somehow area ruled, from the swept back wing, the place of canopy, but the B-58 'Hustler' fuselage is actually 'Area Ruled', you can have a look at it. Think in some USA aircraft cases where Area ruling greatly improved results.

Sometimes the coke bottle shape is necessary to achieve area rule, but not necessarily.  Most fighters are designed around area rule to some degree or other, but you really can't tell just looking at them because the contour volumes are more subtle.

Maybe a little, but not really worth doing.
Area ruling is not a precise science. The exact cross-section used to calculate the area depends on the Mach number, so the best you can do is pick some problem speed and area-rule for that. Also, junctions and other features do not always blend smoothly, so the ideal area profile as you work along the plane is not achievable. You end up making compromises.
And even then, unless your design already has a problem, area ruling will not magically push your plane faster. For example the English Electric Lightning had a similar straight-through fuselage design yet was capable of Mach 2 plus.
The MiG-19 was one of the fastest "pre-area rule" types in large part because it had among the fewest problems with sharp changes in area. The kind of re-contouring that Convair undertook between the YF-102 and YF-102A would not need to be nearly so drastic on the MiG-19 and would not bring anything like the same benefits.
The Mig-21 is in some ways what you suggest, a developed MiG-19 with area ruling (and a few other little tweaks!). Prototypes were originally built with swept and delta wings, and the delta only then selected for production.

About English Electric, if you add enough power, a flat brick will go transsonic, Area Ruling is just a matter of effciency, but your answer is sound. Blessings +


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In the attached image of the Hoffman development of Arup design, the wing is shaped in a somehow anhedral way. Why?
Most airplanes of its days had a Dihedral, or a wing thickness tapering with a 'V' shape, this one is the opposite, hints exist as done to avoid: 'Dutch Roll', or that the ordinary way will impair performance. What is it??

This plane is very interesting as it highlights a few design options.  Firstly, in seems to have neither significant dihedral nor anhedral, but it does have landing gear underneath it, lowering its CG for stability.  Being low aspect ratio, the builder probably decided to forego dihedral as cross wind gusts could make for a rolly, uncomfortable ride.  A slight sweep in the leading edges of the wing would provide some "dihedral effect" when rudder was applied.
Now for the wing itself, not albatross or eagle, we'll go flying squirrel here.  But notice how low the landing speed is.  This is a virtue of lower AR wings, though not as efficient as high AR (albatross) wings, they do stall at significantly lower speeds.  This design was also applied to the Navy "flying flapjack" and it performed well.
And thanks for the article, the ads made good reading too!

Send me everything listed -- look at those balsa prices!

I don't think it's a deliberate anhedral. It's just a geometrical consequence of having an airfoil with flatter bottom. If you imagine a plane connecting the leading edge and the trailing edge, it will be flat. Or likewise, if the airfoil had a completely flat bottom, the lower surface would be flat, and the top one sloping down.

The question focuses on inferior side of Hoffman wing being flat, upper side thinning from root to tips, opposite as the Johnson Circular Wing plan, that probably never flew. In YouTube: 'Wingless wonders'; the Hoffman flies. Johnson's has a flat upper wing side, some kind of a dihedral in the inferior side, close to NASA 'Flying bathtub' lifting body. This dihedral-like, besides providing stability, if too much, it could induce 'Dutch Roll', raise drag, decreases chances of noxious Ground effect on landing, more possible with the flat bottom approach [![R B Johnson Round Wing 1931][1]][1]

Raoul J Hoffman was the engineer who changed the concepts of the podiatrist Snyder into the well flying Arup line of airplanes. No 'visibility' problems exist today for Short AR designs as cost of a TV camera and screen is below that of a full gas tank

These low aspect ratio wings had a vertical speed at landing higher than more common designs, but speed respect to ground was low, this can help in aircraft carriers and some dangerous airports, landing is along an inclined line, not a curve tangent to ground.


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How can I see all commercial destinations to a country from a particular airport?
I live in Las Vegas, which obviously has a large airport with a broad array of international flights. Is there a quick way for me to see all the non-stop commercial flights that are available to Mexico? I don't need to necessarily know the airline, just which cities are available for non-stops from Las Vegas.

I'm voting to close this question as off-topic because this belongs more to [Travel.SE]

@anshabhi if any of that were true then answers wouldn't have been generated. The idiot mods on here prefer to close over dumb rules instead of letting the communities thrive. Don't be a moron and sacrifice results for process

@JimBeam Just because the users here have good answers doesn't mean that it is on topic, or that your question would not also have good answers on another site.

@anshabhi you people are ridiculous

Even I agree with that. The moderation here is really very strict and now they are over to hunt the comments as well. This is an older post and in newer ones, the mods are recklessly deleting even a little off-topic comments. It appears that they want to apply some very harsh standards of quality here, as if they were manufacturing some aircraft... I voted your question as off-topic, because it just appeared to be.. but seriously, we people are "ridiculous".

There can be a number of websites which can provide this information. Wikipedia and the specific airline's website can be a great resource too.

OpenFlights gives you a graphical representation of direct flights from a particular airport. It appears that this service is for all major airports across the globe.
SkyScanner can provide the information you are looking for. As for your example, you'll enter Las Vegas in the From text area, and then leave the To area blank. It will search all destinations (grouped by countries) where there are direct flights (or with one stop) available.

Thank you. Open flights is basically exactly what I was looking for.

Wikipedia entries for airports almost always have the list of destinations served from the airport, and I think the plane-spotter community keeps them very up to date. But it is indexed by airline, not country. Given the likelihood of there being only 2-3 destination airports in a single country, you could search on those city names - or else just browse the list.
The entry for McCarran International in Las Vegas is https://en.wikipedia.org/wiki/McCarran_International_Airport#Airlines_and_destinations

For seeing all non-stop flights, you may use any travel website. See this example using Cleartrip for non-stop flights between LA and Mexico.
And, you can use any flight tracker for all the cities served by an airport. Have the knowledge of airlines and airliner as well! Here are all the departures from KLAX by Flightaware. Alternately, you may check for the airport's website to see all the destinations served by an airport (available at websites of only few airports). For ex. here are all the destinations served by Edinburgh airport.
And, if these don't work out, go with timbo's idea!


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How are evacuation tests made as realistic as possible?
I've seen the video of the A380 evacuation test and similar ones.
While the fast evacuation is impressive, a criticism often heard (and read in the comments on YouTube) is that these people were prepared and (roughly) knew what was about to happen. Also, people in the back of the queue were not in fear of their lives, so they just waited on their turn.
No one tried to go against the stream to look for loved ones seated in different rows, no one tried to get at their personal belongings to try and take those with them.
What measures are taken to make evacuations tests as realistic as possible?
I've heard that incentives are offered to passengers to get out as fast as possible, for instance $100 for the first X passengers to get out, for example in this episode of QI (at the 35:24 mark). Is that true? If not, anything else?
I of course appreciate the conflicting interests that while ideally the test should be as realistic as possible, the test passengers can't be endangered.

Yes, the test is under what some may consider as ideal conditions, however they all are and can be compared to the standard (90 seconds) that way to show that it is possible. An actual evacuation will most likely be slower, but like most things in aviation a safety factor was likely taken in to account when the benchmark was set. If it can't be done in 90 seconds even under ideal conditions, then the problem will be even bigger in an actual evacuation.

On a related note: it is possible to train people to handle the realistic situations. You cannot do that in real life but there's people doing research in virtual reality exactly for that reason. The term used is serious game. My university is doing some research in the field and has already developed an Oculus Rift application to train people on how to handle an emergency situation on an evacuating plane. So: in the future the training could be much more effective using new technologies to provide realistic simulations.

Filling the cabin with non-toxic smoke and adding heaters to simulate fire without telling the participants that was going to happen would certainly help make the testing more realistic.

@FreeMan: What is this "non-toxic smoke" of which you speak?  At high enough density to fill the cabin realistically, you can pretty much guarantee it will be toxic no matter what kind of smoke it is.

@MasonWheeler There are plenty of smoke machines in use at parties/concerts/etc that will produce a non-toxic smoke/fog that will make it hard for the test subjects to see where they're going. Something like these

Arguably the passengers on an actual aircraft being evacuated know what's coming to: They do that safety dance...er briefing...before each flight, and if there's time to give the cabin advance warning (e.g. if there's a known landing gear issue) the flight attendants will re-brief everyone. Still it would be interesting to see these tests repeated with "spoilers" added - people who specifically are told to grab luggage, check on someone in another row, etc.

As the manufacturer has the duty to prove the plane can be evacuated in 90 seconds, you can be sure the passengers have actually been rehearsed to evacuate (but not in the previous 6 months, this is not allowed). That's not a problem however, each aircraft must comply with the 90 seconds limit. An actual evacuation is not expected to be done in 90 seconds. Note that the test is aimed to also test how fast the exits can be opened and the slides can be inflated. All that is done in "night conditions". See Aircraft Evacuation Testing.

The problem with making these tests more realistic is also an ethical one: even under the relatively safe "unrealistic" conditions, about 6% of test subjects sustain injuries from abrasions and bruises up to permanent damage during an evacuation test. And these are not professional test pilots who chose to put themselves in a dangerous situation. Employees are allowed to participate, but they are not allowed to sit near the exits, so at least the portion of the subjects sitting near the exits must be basically strangers off the street. A way to reduce the possibility of injuries would be …

… to use professional evacuation testers the same way professional test pilots are used, but that would of course negate pretty much any sort of realism left, so it is out of the question as well. We want the simulated passengers to be normal people not professional airplane egress specialists.

@FreeMan It's not the sight of the smoke that causes panic, but the suffocating and burning sensation when breathing in extremely hot fine particulate. There is no way to simulate that well without using hot, fine particular which most certainly is not safe. The closest alternative I could think of would be any kind of caustic chemical that causes a burning sensation in the lungs and eyes while not being particularly dangerous, but I doubt that would pass an ethics board review.

The only ‘realistic’ way of carrying out a full scale evacuation of an aircraft would be to put people in it and set it on fire. That said, FAA takes some efforts to keep the evacuation tests as realistic as possible without seriously endangering the safety of the 'passengers'. The FAA Advisory Circular 25.803-1A - Emergency Evacuation Demonstrations lays out some rules regarding this.

Only half the exits are used for evacuation by a representative mix of passengers, with some debris (luggage, pillows etc) strewn in the path.
The aircraft is to have the maximum number of passengers and flight crew, with a representative mix of age and genders with dolls for children.
Not training any of the participants in the test, i.e. both the flight crew and the participants are not trained to evacuate the (that particular) aircraft. According to the Advisory Circular,

The flight attendants should not be trained for specific demonstration conditions, except that specific training should be given which relates to the safety of the participants prior to and during the demonstration.

The participants are clearly told what they are going to do.

The prospective participants should be informed of the purpose of the demonstration and the expected duration of their participation.

Though the participants know 'why' they are there, they are not supposed to know 'when' demonstration is going to start.

Neither the crew nor passengers should hear or otherwise receive any indication that the demonstration is about to begin. The first indication to participants should be the extinguishing of the normal cabin lighting.

The participants should not know about the evacuation plan beforehand.

If safety devices or any other equipment external to the airplane could indicate to the test participants which exits are to be used in the demonstration, passengers and crew should enter the airplane through a tunnel or other means that will prevent them from seeing that indication.

The participants are more than likely paid (employees with previous knowledge of the system are not allowed), though there is no mention of any 'prize' for first getting out. The regulations simply state,

Prospective participants should be informed of any direct benefits to them (e.g., pay, meals, etc.) and of benefits to society

The participants are prevented from interacting with outside and to recieve any information from outside the aircraft.

...exterior windows should be obscured from the outside to prevent both viewing of the outside conditions and any ground lighting from shining into the airplane.

There is intense debate over how 'realistic' the evacuation tests are. For example, the evacuation of British Airtours Flight 28M, a Boeing 737 took more than 5 minutes (resulting in the deaths of more than 50 people), though the aircraft had passed the test.
According to Aircraft Evacuation Testing: Research and Technology Issues, there are multiple problems with this type of testing:

There is only one data point (Full scale evacuation tests are costly; each one costs nearly $1m)
The conditions do not reflect the actual ones experienced (for example, fire, smoke etc.)
The participants know why they are there (Employees of the manufacturer are not barred, though they may not sit near the exits).
The participants do not reflect the actual demographics of the flying public, and if considered, increased the evacuation time significantly (obviously, disabled people and kids are not there in demonstration).
The participants do not experience any trauma. Of course adding smoke or fire is not an option as already around 4-5 percent people are getting hurt.

Image from Aircraft Evacuation Testing: Research and Technology Issues
September 1993 OTA-BP-SET-121 NTIS order #PB94-107620
As a result of the aforementioned BA28M accident, UK's, Cranfield Institute of Technology (CIT) carried out a number of tests which included a case where financial incentives were offered to foster competitive behavior among
test participants. The study determined that,

the presence of a competitive element had a significant impact on egress
  rates for evacuations through the bulkheads, but did not affect the rate of evacuation through the Type III exit

Note: According to FAA, This type is a rectangular opening of not less than 20 inches wide by 36 inches high, with corner radii not greater than one-third the width of the exit, and with a step-up inside the airplane of not more than 20 inches. If the exit is located over the wing, the step-down outside the airplane may not exceed 27 inches.
The US Department of Transportation also carried out studies, which resulted in similar conclusions.

Image from Aircraft Evacuations Onto Escape Slides and Platforms I: Effects of Passenger Motivation, DOT/FAAAM-96/18
The study found that the introduction of smoke did not have much effect, but noted,

Although this research design biased against revealing any deleterious effects of smoke on egress, previous studies using smoke had shown robust reductions in egress speed.

Giving monetary incentives to participants reduced the time required for egress.

Image from Aircraft Evacuations Onto Escape Slides and Platforms I: Effects of Passenger Motivation, DOT/FAAAM-96/18
In this study, the 'competitive' group were offered a financial bonus of $50 to be among the first 25% passengers to evacuate the arcraft. However, the passengers in the competitve trials

...(became) more aggressive and climbing over seats, outmaneuvering other passengers, etc. to get out quickly.

resulting in broken over seat backs, among others. I suspect participant safety is the main reason FAA doesn't offer any incentives duing demonstration.
Because of these reasons, simulations are being increasingly used in modelling evacuations which include variables like crowd behavior, state (of the person), mobility, etc.

This is one of those once-a-month-for-the-whole-network answers that really justifies the existence of Stack Exchange by teaching so much more than merely what was asked. Well done! +1

This answer is also another great example of "Got a question? There's an Advisory Circular for that!"

I love your opening clause...I was just about to comment on the question "Put a fire under 'er!"

"Put people in it and set it on fire" ... If I see Ashton Kutcher at the bottom of this emergency slide, someone will be hearing from my lawyer

"around 4-5 percent people are getting hurt" 4-5% of people participating in the demonstrations?

In the recent case of JAL 516 it ended up taking 18 minutes to evacuate.

IIRC, the passengers that were interviewed in the video you linked to explicitly said that they did not know exactly what was about to happen. They only knew they were going to have to evacuate in the dark with debris on the floor with half of the exits inoperable. They did not know when the evacuation was going to happen; they sat around in a dark plane for an indeterminate amount of time. If your airplane is crashing, you pretty much also know all of that in advance.
Likewise, in a real evacuation, there is an even bigger incentive than a stack of dollar bills to get out fast: your life.
Note that the A380 evacuation certification test actually did lead to one injured test passenger. The most unrealistic part is the vast number of people helping with the evacuation on the ground outside of the airplane. However, those do not affect the speed of the evacuation, only prevent serious injuries (e.g. by getting people off the slides more quickly than in a real evacuation, preventing a possible "pile-up" with passengers slamming into one another at high speed).
Some of them look pretty well disoriented to me in the IR videos.

Of course, the incentive is bigger in a real evacuation, but that is my point. Passengers in the back will wait in a test, but may be panicking in real life and trying to get out as fast as possible. Also, they may try to look for friends seated in a different row or try to bring personal belongings with them. I'll try editing my question to reflect this.

Two points to add: There are requirements from the authorities (FAA/EASA) about the distribution of passengers - you must have a certain number of passengers over a certain age, some must be carrying pillows (simulating infants), etc. Also, even though the test participants knew there was going to be an evacuation, they didn't know when it would happen exactly (they were sitting around in the dark plane for a while), and they didn't know which exits would be available.

I'm not quite sure people helping on the ground do not affect the evacuation. You may hesitate a fraction of seconds when seeing the pile of bodies at the bottom of the escape slide may. Multiply the fraction of seconds by the number of passengers for each door and it may slow down the evacuation by few seconds.

@ManuH I suspect hesitating in the face of a pileup would only be significant in non-time critical evacuations.  If the plane's on fire/sinking the mob pressure'd be too high for anyone to pause at the threshold.

@Lightsider: thanks, I meant to mention that the "passengers" (nor the flight attendants, I might add) didn't know when exactly the evacuation would happen, but I forgot. It is implicit in the "they only knew", but deserves an explicit mention.

What measures are taken to make evacuations tests as realistic as possible?

The maximum possible number of "crew" and "passengers" are used.
There are rules about the mix of ages and sexes.
The "passengers" are not briefed or trained in advance.
They must not have participated ina similar exercise in the last 6 months
The aisles and exit routes are strewn with luggage
I think it is typical carry-on luggage. About half the
likely contents of overhead storage in a full plane.
Half the exits are declared inoperative
There are financial incentives to get out quickly
Passengers are prevented from seeing what is happening outside (I think)

Could you add any references?

In addition to the other excellent answers, the simulator is equipped with speakers, extra lights, and a fog machine - or at least one airline has such a simulator and I expect others do also. I have done one simply for fun, we were not being timed, and the sound effects, including screaming and weird roaring whooshing noises, plus the orange fire-like light from an unexpected direction and the presence of "smoke" that luckily doesn't smell of smoke all got the adrenaline going and made things more challenging (and therefore realistic) than they otherwise would have been. And we knew exactly what we were doing, we had been offered the treat of experiencing a simulated evacuation.
As for the thing about going backwards - an airline could always instruct someone to misbehave like that, but one of the reasons the target time is set to 90 seconds with only half the exits in use is so that even if such misbehaviours double the exit time, you still get everyone out in 3 minutes. And in real life you have staff who could tell people not to do that.
(Or other passengers: 30 years ago news reports about an evacuation from a soon-to-be-burning plane included tales of the man with the big deep voice who called people over to the exits and urged them to leave during the 90 seconds they got before the smoke turned to flames and those still on board all died, including the folksinger famous for his big deep voice. Two bodies were found far from the exits even though all passengers had moved close to the exits during the descent; it was concluded they had gone past the exits in panic. This is why some airlines instruct you to count rows to the exit from your seat. This flight is also why there are smoke detectors in the lavs.)

May be paid too, in mock drill aircraft manufacturer/airliner wants to show this as added advantage for how fast evacuation can be done in that particular model of aircraft. If they have not paid for this activity, then passengers might not be as quick as you can observe them in video.
Even in software companies, when they do Fire mock drill, you can see employees coming out of the company slowly and record the time with very slowest evacuation.
Coming to point "test should be as realistic as possible" - you may really need to simulate the evacuation (may be accident/emergency) to continue this evacuation

In case if accidents

For passengers, it might not be easy to cross all the possible hurdles created by accident
Passengers near the evacuation doors are not giving up the way
Emergency Doors might not be opening as quick as you will able to do it in mock drill
Accidents will not cause same damage/scenario each time it occurs

With all these reasons you will not be able to evacuate passengers as fast as you do it in mock drills.
I don't know how (on what basis) this particular equipment is certified.

passengers grabbing their carry-ons

This doesn't answer the question, which is how are the drills made realistic.